A very powerful electro-magnet would do the trick but I think it would be very hard to make it small enough to fit in that form factor. With a metal computer case and a the metal hard drive in the way it would be very hard to magnetically change the data on the disk but it is possible.

I read somewhere that the magnets used to write to a hard drive are amazing strong, but so small that they are only effective for a few millimeters around them. I think your goal is a hard thing to reach, but if you look for some of the data compiled for 'what if' terrorist attacks, you'll probably find alot about EMP and the risk such devices pose. Your proabably more likely to damage the surrounding electronics than wipe an HDD